In the manufacturing of disposable absorbent articles, such as diapers, adult incontinence articles, and sanitary napkins, it is a common manufacturing practice to initially fabricate a plurality of individual articles, such as diapers, that are serially spaced in interconnected positions on a continuous web. Individual diapers thereafter are formed by directing the web into a rotary cutter to cut the web between the spaced pads, usually as the final cutting operation in the manufacturing process.
In order to produce uniform diapers, and to eliminate waste, it is desirable to control relatively precisely the location at which the web is severed (typically referred to as the "final cut"). It is particularly desirable to cut the web in a correct position relative to features that are formed on the diaper. For example, disposable diapers frequently use adhesive tab strips and it is desirable to control the distance between these tabs and the top of the diaper within a relatively narrow tolerance.
Controlling the location of the final cut relative to other diaper features in an economically viable way has been and continues to be problematic for diaper manufacturers. Variation in the placement of the knife cut is inherent in the diaper manufacturing line equipment due to a number of factors including equipment wear, variations in raw materials, etc. Such variation in the final cut results in sequential pads that alternate between long and short lengths. Web tension and temperature variations also occur when the pull rolls in the manufacturing line expand or contract as the line is placed into production. The increase in temperature makes the roll softer, allowing the web to compress the rolls. As the rolls are compressed, the surface of the roll increases, increasing the circumference of the roll. This results in an increase in the web speed through the roll. The diaper pads are advanced into the final knife with the increased effective pull roll speed, and as a result, beginning from a cold start, the final cut distance can shift significantly.
The variation in the final knife placement may steadily trend from one side of the specification limit to the other in a smooth transition, therefore making the detection of unacceptable variation difficult. Detecting unacceptable variation was previously left to the operators of the production line. This has been proven to be unacceptable in many instances because the variation can go undetected. Even when the variation can be detected by the line operator, it is usually at a point in time where an unacceptable level of product must be scrapped. Furthermore, inefficiencies are created because once the unacceptable variation is detected, the operator must manually adjust the system to correct the final knife placement.
Automatic registration control systems have been employed in web processing apparatus to overcome the problems of manual detection. For example, automatic registration control systems have been used in the manufacturing of price tags. In these systems, the tag web being fed is cut while its cut position is constantly corrected by a rotary blade. The blade is adjusted so that it can cut a variety of price tag webs having different cutting pitches or intervals. In these systems, after a tag is detected having a new cutting distance, the cutter is adjusted to correspond to the correct cutting position. This type of system has not been used in controlling the knife placement along the diaper web because the systems were not designed to accommodate extensible webs used in diaper production, but rather were designed as a means for adjusting the knife when a new cutting pitch or interval was necessary.
Attempts also have been made to control the rate at which a diaper web is fed to the cutter. However, this type of registration control system is undesirable because it requires multiple sensors located along the web, increasing the complexity of the registration system. In addition, adjusting the web speed produces additional tensile stresses on the web by constantly changing the speed at which it is fed to the cutter. Considering that web tension problems are already inherent in diaper manufacturing lines, this problem is compounded by control systems which maintain the registration by continually adjusting the web speed.
Therefore, there exists a need for an automatic registration control system that controls the placement of the final cut with respect to a pre-selected feature of the diaper which eliminates the problems associated with previous registration control methods.